Add simple delete for MANY_MANY

[garex]

I suggest to add just now simple delete logic to MANY_MANY relation. Not waiting for implementing of link/unlink.

Logic there is straight enough: remove in connection table all records, that related to owner. For example in case of saving article with tags we should just delete from article_tag all records, where article_id = 123. It will be enough for all cases, as later insert command adds all really needed records.

In the meantime we have tests written in AntonTyutin/yii-with-related-behavior@e00fc07 fork.

ps: I have read all related issues and comments

[garex]

Closed in favor of pull request #32